Single Metalens Focuses all Colors of the Spectrum in one Point

Metalenses are flat surfaces that use nanostructures to focus light. These simple, flat surface lenses could replace bulky, curved lenses currently used in optical devices.

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a single lens that can focus the entire visible spectrum of light — including white light — in the same spot and in high resolution. This has only ever been achieved in conventional lenses by stacking multiple lenses.

This flat metalens is the first single lens that can focus the entire visible spectrum of light -- including white light -- in the same spot and in high resolution. It uses arrays of titanium dioxide nanofins to equally focus wavelengths of light and eliminate chromatic aberration. Courtesy of Jared Sisler/Harvard SEAS.
Focusing the entire visible spectrum and white light has been a challenge because each wavelength moves through materials at different speeds. Red wavelengths will move through glass faster than blue, so the two colors will reach the same location at different times, which result in different foci. This creates image distortions known as chromatic aberrations.

Cameras and optical instruments use multiple curved lenses of different thicknesses and materials to correct these aberrations.

"Metalenses have advantages over traditional lenses," said Harvard SEAS researcher Federico Capasso. "Metalenses are thin, easy to fabricate and cost effective. This breakthrough extends those advantages across the whole visible range of light. This is the next big step."

The metalenses developed by Capasso and his team use arrays of titanium dioxide nanofins to equally focus wavelengths of light and eliminate chromatic aberration. Previous research demonstrated that different wavelengths of light could be focused but at different distances by optimizing the shape, width, distance, and height of the nanofins. The researchers created units of paired nanofins that control the speed of different wavelengths of light simultaneously. The paired nanofins control the refractive index on the metasurface and are tuned to result in different time delays for the light passing through different fins, ensuring that all wavelengths reach the focal spot at the same time.

"One of the biggest challenges in designing an achromatic broadband lens is making sure that the outgoing wavelengths from all the different points of the metalens arrive at the focal point at the same time," said researcher Wei Ting Chen. "By combining two nanofins into one element, we can tune the speed of light in the nanostructured material, to ensure that all wavelengths in the visible are focused in the same spot, using a single metalens. This dramatically reduces thickness and design complexity compared to composite standard achromatic lenses."

The researchers aim to scale up the lens, to about 1 cm in diameter. This would open a whole host of new possibilities, including applications in virtual and augmented reality.